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Building the Survivor: More TX/RX Switching Stuff

Okay, I’m back at it after taking a little bit of a break on this build. This installment covers more stuff that happens when the Survivor is switched between TX and RX (and from tune mode and back). There’s a bunch of circuitry that controls muting/unmuting the microphone input under different circumstances. I took the time to check out what the TINY13A output pins were doing when in RX, TX, and tune modes, and I discovered a mistake on the schematic. The connections to pins 6 and 7 of the TINY13A (U5) are reversed. pin 6 connects to Q11 and Q13, and pin 7 is the ENTX output. It’s only the schematic that’s wrong–the board layout is correct.

So, on U5, pin 2 switches the VFO and BFO between the two mixers when switching between TX and RX. Pin 3 is where the sidetone and tuning tone is generated, and it’s only generated when transmitting in tune mode or CW mode. Pin 5 takes the input from the PTT switch. Pin 6 mutes and unmutes the microphone input (muted on RX and on TX when in tune or CW mode). Pin 7 turns on the TX amplifier on transmit and turns them off on receive.

For this part of the circuit, install the following parts:

R11, R14, R17, R21, R23, R25, R50

C16, C19, C23, C27, C28, C31

D5

RT2

Q3, Q6, Q7, Q10, Q11, Q13

Let’s talk about TX switching first. When the PTT button is pressed, U5 pin 5 is grounded, and this causes U5 pin 7 to go high. Pin 7 is connected to the gate of MOSFET Q6. Recall that when the gate of a MOSFET is grounded, no current flows from drain to source, effectively shutting off the device. When U5 pin 7 goes high, so does the voltage on the gate of Q6, allowing current to flow in the MOSFET and connecting the base of Q3 to ground through R17, turning on Q3 and providing voltage to the TX amplifier circuitry. You can see this for yourself by measuring the voltage at VTX on the schematic (where R23 connects to Q3–on the board, it’s the end of R23 that’s closest to Q3). When in TX, this voltage should be about 12V (whatever the battery voltage is). On RX it should be zero.

Voltage from VTX is fed via R23 to a voltage regulator circuit made up of D5 (a 5.1V zener diode) and C23, when then connects to the collector of Q10 (look for “V_TX” on the schematic). Q10 is the microphone buffer amplifier. When transmitting voice, U5 pin 6 is low. This shuts off Q13 and isolates the base of Q10 from ground, allowing the MIC input to drive the output of Q10. On RX, U5 pin 6 is high, causing Q13 to conduct, connecting the base of Q13 to ground and effectively muting the MIC input. U5 pin 6 stays high on TX in tune mode, too, keeping the MIC muted during tuning and CW. You can see this in action by measuring the voltage at the MIC input in TX and RX modes. In RX and tune modes, MIC should be grounded, but in TX, MIC should be about 1.4V.

One thing I didn’t detect until much later in my build was that in my kit the MIC voltage was much lower (about 0.45V), and my transmitted audio (via the mic) was distorted and uncopyable. A few others on the Hendricks_SSB Yahoo Group were having the same issue. No one has come forward with an explanation for this variation, but with a little trial and error I determined that if I reduced the value of R25 to about 15K, that raised the MIC voltage above 1V and gave me decent transmitted audio. The issue here is that Q10 is a common collector amplifier. Its purpose is not really to amplify so much as to buffer and also to convert impedance. In this sort of amplifier, the emitter voltage is always about 0.6V below the base voltage, so if the base voltage is only 0.45V to begin with, there won’t be any appreciable output. Raising the base voltage of Q10 to 1.4V or so was necessary to get Q10 to do its job, and altering the value of R25 was what it took to get the right base voltage.

Finally, U5 pin 6 also controls Q11, which is effectively a switch to connect/disconnect the audio oscillator to the balanced modulator U2. U5 pin 6 is high if the radio is in RX or tune mode. In either case, this causes Q11 to conduct and connect the audio oscillator to Q10. Note that when the radio is in RX mode, the audio oscillator isn’t active, so it’s only when the rig is in tune mode that the audio from the oscillator gets fed to the balanced modulator.